1. Field of the Invention
The present invention relates generally to resistive inks which are used as the resistive element in potentiometers. More particularly, the present invention relates to resistive inks which are suitable for use in low moisture and high vacuum environments.
2. Description of Related Art
Potentiometers having an electrical contact wiper which rubs against a resistive element are well known, as described, for example, by G. J. Gormley in the publication entitled "Conductive Plastic Film Precision Potentiometers" in Electronic Engineering Times, Issue 282, Mar. 26, 1984. The resistive elements in potentiometers were originally windings of nichrome wire. More recently, these wire-wound resistive elements have been replaced with plastics which are treated to become semi-electro-conductive. The semi-electro-conductive plastics are typically applied to a substrate to form a semi-conductive coating. These plastic coatings are referred to as "resistive inks."
Carbon black has been widely used as an additive which gives the plastic resistive ink its desired semi-electro-conductive character. A problem with the use of a carbon black is its characteristically abrasive nature. The isotropically hard carbon particles cause excessive wear of the mating potentiometer contact as it slides over the resistive ink.
In the past, the abrasiveness of carbon black has been mitigated by blending small amounts of a highly crystalline graphite into the carbon black filled polymer-based resistive ink. The graphite functions as a lubricant because of the preferential accumulation and orientation of low friction graphite basal planes on the ink's surface, due to sliding of the metal alloy wiper contact across the ink surface. As a result, friction and wear of the precious metal contact wire and resistive ink are reduced. This reduction in friction and wear results in low electrical noise and extends the life of the potentiometer. Even though graphite has anisotropic conductivity, the small amounts (typically less than about five weight percent) added to the carbon black filled resistive inks does not adversely affect the electrical performance of the ink. Accordingly, such carbon black filled resistive inks have become quite popular for use in a wide variety of potentiometer applications.
The solid lubrication mechanism of graphite is effective in ambient air, which normally contains more than a 25 volume percent relative humidity. It is believed that the moisture present in the air provides intercalation of the crystalline graphite which renders it a good solid lubricant. However, graphite lubricated potentiometers which are subjected to a vacuum environment become at least as abrasive as the original carbon black filled resistive ink. As a consequence, such resistive ink-operated potentiometers designed for spaceborne uses will have a relatively short operational life due to high rates of wiper abrasion and wear.
As is apparent from the above, there presently is a need to provide improved resistive inks for use in spaceborne potentiometers, wherein the resistive ink will remain lubricated in the high vacuum and low moisture environment of outer space.